Relative scan feed motions of object bearing devices toward scanner units of computed tomography systems, which are usually referred to as a so-called feed, are usual in the daily routine of computed tomography system operation. This motion should be distinguished from the scan rotational motion of an X-ray radiation source and an X-ray detector of the computed tomography scanner, which rotate around an object bearing device with an examination object borne thereon, e.g. an organ of a human body. The scan feed motion is generally directed perpendicularly to the rotational plane defined by the scan rotational motion.
In a helical tomography scan, an examination object is continuously moved relative to the encircling annulus of the X-ray radiation source during an irradiation. Hence, the scan rotational motion and the scan feed motion are combined such that the examination object is irradiated in a helical shape.
Here, the feed is used to be able to ensure complete coverage of an examination object. This is particularly necessary in the case of relatively large examination objects combined with detection regions which cannot acquire the whole examination object at once. During the feed, either the object bearing unit or the scanner unit is moved in the direction of the respectively other element, or both units are moved toward or away from each other.
In general, such a relative scan feed motion is a linear motion, i.e. it is effected at a constant speed and unchanging motion direction. However, in specific cases, e.g. a repeated helical scan by a computed tomography system, the scan feed motion can be repeated a number of times, i.e. the object bearing device and/or the scanner unit are retracted into their initial position once a run-through is complete and the motion process is repeated iteratively a number of times.
However, in the case of tomography scans, in particular computed tomography scans, it can often be the case that external or internal interference becomes noticeable during the scan, as a result of which parts of the acquired image data, or image data which can be acquired, cannot be utilized or can only be utilized badly. By way of example, such interference is due to an operator wishing to influence proceedings during a tomography scan or other influences which specifically depend on the examination object.
These are predominantly motions of the examination object or its surroundings, e.g. insuppressible organ motions of a patient, predominantly the heart rate or lung motion, which are both transferred to other regions of the body as well. In the case of a regular cyclical motion such influencing factors can be artificially eliminated during the tomography scan or in the following procedures. This is effected by way of so-called prospective or retrospective “triggering” or “gating”. In both methods, time corridors are fixed during which the cyclical motion is in a phase of relative rest, e.g. in the transition from respiration to aspiration in the breathing curve or during the first half of the diastole in the cardiac curve. In the case of prospective triggering or gating, raw image data is only acquired within such time corridors, whereas in retrospective triggering or gating only those parts of the generally continuously acquired raw image data which were acquired during such time corridors are continued to be used for imaging optimized processing.
By contrast, in the case of irregular motion, there is only limited scope for such a correction by triggering or gating and so there is the risk that parts of the acquired image data are worthless for the further examination. In particular, the feed during the scanning procedure within the scope of a helical tomography scan leads to only a very limited amount of raw image data being acquired for a certain tissue volume. If such an irregular motion occurs at just that time when a particular tissue region is scanned, it may be the case that no usable image data is available for this region. This problem is increased by the fact that for example the heart rate frequency can in any case change during a scan. The tomography system cannot react to this and therefore the risk of unusable acquired image data is further increased.
DE 10 2006 056 997 A1 proposes that the feed speed of a patient couch during a CT spiral scan is changed in accordance with the changes of a heart rate of a subject lying on the patient couch.